Submission #169288

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Source Code Expand

#include <iostream>
#include <vector>
#include <cstdio>
#include <algorithm>
using namespace std;

int main() {
  int N;
  while (cin >> N) {
    double sum = 1.0;
    vector<long long> C(N), idx(N);
    for (int i = 0; i < N; ++i) {
      cin >> C[i];
      idx[i] = i;
      sum *= (i+1);
    }

    if (N > 8) {
      cout << "0" << endl;
      continue;
    }

    long long cnt = 0LL;
    do {
      for (int i = N-1; i >= 0; --i) {
        int flip = 0;
        for (int j = i-1; j >= 0; --j) {
          if (C[idx[i]] % C[idx[j]] == 0) {
            ++flip;
          }
        }
        if (flip % 2 == 0) {
          ++cnt;
        }
      }
    } while (next_permutation(idx.begin(), idx.end()));

    printf("%.8f\n", cnt / sum);

  }
  return 0;
}

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